Method and apparatus for controlling flare engagement height in automatic landing systems
FIELD OF THE INVENTION 
     This invention relates to aircraft automatic landing systems and, more particularly, to methods and apparatus for initiating flare during an automatic landing procedure. 
     BACKGROUND OF THE INVENTION 
     Various automatic landing systems have become available during the past several years, often being incorporated in the aircraft&#39;s autopilot or flight control system. Basically, these automatic landing systems are digital signal processors that execute control laws that cause the aircraft to follow glideslope and localizer signals so that the aircraft approaches the destination runway without manual control by the pilot. When the aircraft reaches a suitable point in its descent, the automatic landing system initiates a flare maneuver to arrest aircraft rate of descent (sink rate) so that the aircraft touches down on the runway at a proper position and at a suitable sink rate. In the past, the point at which the automatic landing system initiates the flare maneuver has been based on the aircraft attaining a predetermined altitude. The flare initiation altitude used in these systems depends on the flight characteristics of the aircraft employing the system and is established to allow the automatic landing system to bring the aircraft to a safe landing in all situations in which the automatic landing system is used (i.e., for all ground speeds, aircraft landing weights, sink rates, etc., that are within the aircraft&#39;s automatic landing profile). For example, it has been typical practice to initiate an automatic landing flare maneuver when the aircraft landing gear is 50 or 45 feet above the ground, depending upon the particular aircraft of interest. 
     Initiating automatic landing flare maneuver at a predetermined altitude has certain disadvantages and drawbacks. For example, when an automatic landing is executed in a tail wind, with a heavily loaded aircraft, and/or is executed at a high-density altitude destination (which refers to the altitude of the destination airport adjusted to reflect ambient temperatures greater than standard atmospheric temperature), the aircraft groundspeed is higher than the nominal ground speed for the automatic landing procedure. This means that the aircraft will have a higher than nominal approach sink rate (i.e., will descend at a rate higher than it would during an approach at a nominal ground speed). To arrest the relatively high sink rate, it may be necessary for the automatic landing system to exert fairly aggressive control of the aircraft during the flare maneuver. Moreover, in situations in which the aircraft sink rate cannot be decreased to a low level, the aircraft may make a relatively hard landing. Although automatic landing systems in current use safely land the aircraft under relatively high ground speed approach conditions, the required flare maneuver and/or resulting hard landing may cause an undue degree of passenger concern, apprehension and discomfort. 
     Initiating an automatic landing flare maneuver at a predetermined altitude also is somewhat disadvantageous when the aircraft executes the automatic landing procedure at a relatively low ground speed because of head winds or a lightly loaded aircraft. When the aircraft ground speed is relatively low, the aircraft descends along the approach path with a lower than nominal approach sink rate. When this occurs, the automatic landing system may control the aircraft in a manner that results in relatively long flare time causing the aircraft crew and passengers to sometimes experience a &#34;floating&#34; sensation. This may cause the pilot to mistakenly believe that the landing will require more than normal runway distance (i.e., the aircraft will land &#34;long&#34;). Such a perception on the part of the pilot can cause unnecessary pilot concern and increases the possibility that the pilot will unnecessarily assume manual control of the aircraft, even when the landing is being executed under low visibility conditions. 
     For the above reasons, a need exists for methods and apparatus that initiate automatic landing flare maneuver in a manner that better accommodates higher than nominal and lower than nominal approach speeds (i.e., ground speeds that are within system limits, but higher or lower than optimal). 
     SUMMARY OF THE INVENTION 
     Provided in accordance with this invention is a method of and apparatus for scheduling the altitude at which flare is initiated during an automatic landing system on the basis of the aircraft ground speed during the landing approach. In effect, in the practice of the invention, the range of ground speeds for the automatic landing system that incorporates the invention is partitioned into: a high ground speed range; a nominal ground speed range; and, a low ground speed range. The nominal ground speed range is selected to encompass landing conditions under which the aircraft executes a relatively soft landing without substantial maneuvering during the aircraft flare procedure and without exhibiting a relatively long flare time. The flare height established by the invention for this nominal ground speed range is a constant landing gear altitude, which generally corresponds to the flare height that would be used under the conventional technique of executing a flare maneuver at a predetermined altitude (e.g., landing gear altitude of 45 or 50 feet). In accordance with the invention, flare height for aircraft ground speed above the nominal ground speed range (i.e., ground speeds within the high ground speed range) is increased as a function of increasing ground speed and flare height for ground speeds below the nominal ground speed range (i.e., ground speeds within the low ground speed range) is decreased as a function of decreasing ground speed. 
     In accordance with the method of the invention, a signal representative of aircraft ground speed during the approach phase of an automatic landing procedure is processed to provide a signal representative of what is referred to herein as the aircraft &#34;adjusted approach sink rate.&#34; As shall be described in more detail, the adjusted approach sink rate in effect corresponds to the average expected sink rate of the aircraft while it is descending along a glideslope beam with compensation being included for deviations from the desired glideslope path (i.e., short term deviations from the glideslope angle that are caused by atmospheric disturbances such as vertical wind gusts). In accordance with the invention, the signal representative of aircraft adjusted approach sink rate is processed to automatically and linearly increase landing flare height as a function of increasing adjusted approach sink rate when the magnitude of the adjusted approach sink rate exceeds a first predetermined value (i.e., when the landing approach ground speed is within the above-defined high ground speed range. Flare height is maintained constant (e.g., at 50 foot landing gear altitude) when adjusted aircraft approach sink rate is less than the first predetermined value and greater than a second predetermined value, i.e., when aircraft ground speed is within the nominal ground speed range. For adjusted approach sink rates that are below the second predetermined value (i.e., sink rates that correspond to ground speeds in the low ground speed range) the signal processing of the invention provides a flare height that linearly decreases relative to the decreasing adjusted approach sink rate. Preferably, the maximum and minimum flare heights that are generated in accordance with the invention are limited to predetermined upper and lower limits. For example, in one realization of the invention, the maximum flare height is limited to 60 feet; the minimum flare height is limited to 40 feet; and the flare height for the nominal range of adjusted approach sink rates is established at 50 feet. 
     Under the currently preferred practice of the invention, the signal representative of the aircraft adjusted sink rate is filtered to eliminate signal noise prior to implementing the signal processing that produces the flare height initiation signal. In the disclosed method and apparatus, the filter employed is a recursive digital filter that results from a bilinear transformation of the frequency domain transfer function for a first order lag circuit. 
     It has been found that practice of the invention substantially alleviates previously mentioned disadvantages and drawbacks of automatic landing systems that employ a constant flare height for all landing conditions. In this regard, using an aircraft simulator to execute simulated automatic landings under landing conditions that had been encountered during various aircraft test flights, it was determined that the invention provides substantial improvement both for relatively high ground speed automatic landings and relatively low ground speed automatic landings. In these flight simulations, when a constant flare height was utilized during high ground speed landings, the aircraft sink rate when the aircraft touched down varied between approximately -5.6 and -11.5 feet per second. When the invention was incorporated in the same automatic landing simulations, the aircraft sink rate at touchdown significantly decreased, ranging between approximately -1.8 to -7.1 feet per second. During simulated landings under low approach ground speeds, the average time required for the aircraft to execute the flare maneuver decreased from approximately 11 seconds to 8.5 seconds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects and advantages of the invention will be more fully understood by reference to the following detailed description when taken in conjunction with the accompany drawings in which: 
     FIG. 1 graphically depicts the flare height-adjusted sink rate relationship that is established under the practice of the invention; and 
     FIG. 2 is a block diagram that depicts the signal processing effected by the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 graphically indicates the manner in which the currently preferred embodiments of the the invention schedule automatic landing flare height as a function of aircraft ground speed during the approach phase of an automatic landing procedure. More specifically, FIG. 1 depicts the relationship between the altitude or height at which the flare maneuver of an automatic landing is initiated relative to the aircraft adjusted sink rate. As shall be described relative to FIG. 2, adjusted approach sink rate is derived from the aircraft ground speed during the landing approach (prior to initiation of the flare maneuver) and glideslope deviation rate. 
     In FIG. 1, the automatic landing system flare height ranges between a lower limit, FH MIN , and an upper limit of FH MAX . For sink rates of a magnitude greater than a first predetermined value SR U , flare height is maintained at the upper flare height limit FH MAX . For adjusted approach sink rates of magnitude less than the first predetermined limit SR U  and greater than a second predetermined limit SR 1 , flare height linearly increases as a function of increasing adjusted approach sink rate (i.e., a more rapid rate of aircraft descent results in greater flare height). As is indicated in FIG. 1, sink rates of a magnitude greater than SR 1  and less than SR U  fall within a &#34;high ground speed&#34; range in which the landing approach is being made at a ground speed that is within the operational limits of the automatic landing system, but which may result in a fairly aggressive flare maneuver and/or may result in a landing that is harder than desired. For purposes of reference, the high ground speed range can be considered as extending between a ground speed GS.sub. U and a lower ground speed GS 1 . However, it should be noted that is a specific relationship between the value of aircraft adjusted sink rate and aircraft ground speed exists only under conditions in which the landing is being made in the absence of atmospheric disturbances such as vertical wind gusts that cause the aircraft to deviate from the glideslope beam. 
     With continued reference to FIG. 1, flare height is constant at a nominal flare height value of FH NOM  for all sink rates of a magnitude less than SR 1  and greater than SR 2 . This region of constant flare height, which extends between reference ground speeds GS 1  and GS 2 , encompasses adjusted approach sink rates (and, hence, ground speeds) under which the automatic landing system will execute a relatively smooth flare maneuver and guide the aircraft to a relatively soft landing from a constant flare height. Adjusted approach sink rates of magnitude less than SR 2  and greater than a lower limit SR L  are within the low ground speed range of the flare height schedule shown in FIG. 1. In the low ground speed range, which extends between reference ground speed GS 2  and a lower ground speed GS L , flare height decreases linearly with decreasing magnitude of adjusted approach sink rate. Flare height remains constant at FH MIN  (the flare height value attained when the magnitude of the adjusted approach sink rate is SR L ) for all values of SR L  that are of magnitude less than SR L  and are within the operational ground speed range of the automatic landing system. 
     As is illustrated in control law format in FIG. 2, the signal processing that is used in the currently preferred embodiments of the invention basically includes: (a) generating the signal representative of the adjusted approach sink rate (indicated by adjusted approach sink rate generator 10 in FIG. 2); (b) filtering the adjusted approach sink rate (indicated by filter 12); and (c) processing the filtered adjusted approach sink rate signal to determine whether the automatic landing is being made under high ground speed, nominal ground speed or low ground speed conditions and, based on such determination, establishing the flare height at which the flare maneuver will be executed in accordance with the schedule shown in FIG. 1 (indicated by flare height controller 14 in FIG. 2). 
     Turning first to the signal processing indicated by adjusted sink rate generator 10 of FIG. 2, during the landing approach and all times prior to initiating flare, a signal representative of the current aircraft ground speed is multiplied by a signal representative of the glideslope path, which typically is on the order of 3° (indicated by multiplier 16 in FIG. 2). As is indicated by divider 18 and block 20, the product of the glideslope angle and ground speed signals is then divided by a constant (57.3). As is indicated by adder 22, the signal produced by the division step (divider 18) is subtracted from a signal representative of the aircraft glideslope deviation rate. Such signals are typically available in automatic landing systems, with the currently preferred embodiment of the inventions employing a complementary filtered glideslope deviation rate signal. 
     It will be recognized that the signal processing discussed relative to sink rate generator 10 provides an adjusted approach sink rate signal of the form -[(GLS×GS)/57.3]+GSD, where GS represents aircraft ground speed, GSD represents the glideslope deviation rate, the constant (57.3) converts the glideslope angle to radians, and, GLS represents the glideslope angle. When this adjusted sink rate signal is multiplied by the aircraft ground speed during the landing approach, an accurate estimate of the magnitude of the aircraft&#39;s average rate of descent (sink rate) is obtained. The minus sign in the above set forth expression for adjusted approach sink rate denotes that the aircraft is descending rather than climbing and is obtained in adjusted sink rate generator 10 of FIG. 2 by supplying the signal obtained in the division step (by divider 18) to a subtractive input terminal of the depicted adder 22. 
     Since the arrangement of adjusted sink rate generator 10 combines the average sink rate signal with the glideslope deviation rate signal (which is representative of aircraft deviation from the glide path in feet per second), it can be recognized that the adjusted approach sink rate signal takes into account short term deviations from the desired glide path. Thus, when the adjusted approach sink rate signal is processed in the manner hereinafter described relative to flare height controller 14 of FIG. 2, any deviation from the glideslope path that occurs when the aircraft is executing a relatively high ground speed automatic landing will result in an additional increase in the flare height signal produced by the invention. That is, if external disturbances such as wind gusts cause a short term deviation from the desired flight path angle, flare will be initiated at a somewhat higher altitude, even when the external disturbances do not cause a change in ground speed. Similarly, when the aircraft is executing an automatic landing at a relatively low ground speed, the invention operates to decrease the flare height, even under conditions in which the external disturbances do not change the aircraft ground speed. 
     Filter 12 of FIG. 2 is a control law representation of the bilinear transformation of a first order lag circuit, i.e., a circuit having a frequency domain transfer function of K/(s+K), where s is the Laplacian operator and K is a constant. As will be recognized by those skilled in the art, filter 12 is a recursive digital filter, which easily can be implemented in a digital flight control system. During each signal processing iteration, the current value of the adjusted approach sink rate (provided by sink rate generator 10) is summed with the value of the adjusted approach sink rate during the next most antecedent iteration. This operation is indicated in filter 12 of FIG. 2 by adder 24, the input terminals of which are connected for receiving the adjusted approach sink rate signal supplied by sink rate generator 10 and a signal representative of the last most antecedent value of the adjusted approach sink rate signal (represented by the Z transform symbol shown in block 26. The sum of the current and next most antecedent values of the adjusted approach sink rate signal is then multiplied by a first constant C 1  (indicated by multiplier 28 and block 30 in FIG. 2). The next step of the filtering process is subtraction of a signal representative of the output signal supplied by filter 12 during the next most antecedent iteration period. This is indicated in FIG. 2 by Z transform block 34, which is connected between the output of filter 12 and a subtractive input of an adder 32, which has an additive input connected to multiplier 28. The signal that results from the subtraction step is then multiplied by a second constant C 2  (indicated by multiplier 36 and block 38) and summed with the value of the filter output signal that was obtained during the next most antecedent signal processing iteration (indicated in FIG. 2 by interconnection of Z transform block 34 and multiplier 36 with the two input terminals of an adder 40). 
     It will be recognized by those skilled in the art that the constants C 1  and C 2  discussed relative to filter 12 of FIG. 2 relate to the frequency domain transfer function on which the filter arrangement is based and the iteration period of the signal processing system (the computation period used by the automatic landing system for each determination of a filter output value). In the currently preferred embodiments of the invention the frequency domain transfer function for the first order lag effected by filter 12 is 5/(s+5). For this transfer function, the digital filtering constant C 1  is 0.5 and the digital filtering constant C 2  is given by the expression 1-exp(-5 ), where DT is the signal processing iteration period used in the filtering operations, and exp(-5 DT) denotes raising the natural logarithm to the exponential power (-5 DT). 
     As was previously mentioned, the filtered adjusted sink rate signal provided by filter 12 of FIG. 2 is processed to determine whether the automatic landing system is being made under the previously discussed high ground speed, nominal ground speed or low ground speed conditions and, based on that determination, establishing flare height in accordance with the schedule shown in FIG. 1. High ground speed unit 42 of the flare height controller 14 shown in FIG. 2, illustrates the signal processing that is performed when an automatic landing is made under high ground speed conditions (aircraft adjusted approach sink rate magnitude less than or equal to SR U  and greater than or equal to SR 1  in FIG. 1). During high ground speed conditions, the invention operates to add a signal representative of SR 1  to the adjusted approach sink rate signal that is supplied by filter 12. In FIG. 2, this is indicated by adder 44 of high ground speed unit 42 which is connected for receiving the adjusted approach sink rate signal and a signal representative of SR 1  (indicated by block 46). The signal that results from the addition then is multiplied by a constant, DS 1  (which is indicated by a multiplier 48 that is connected in FIG. 2 for receiving the signal supplied by adder 44 and a signal representative of DS 1  (indicated by block 50)). As is indicated by adder 52 in FIG. 2, the signal that results from the multiplication (supplied to a substractive input of adder 52 by multiplier 48) is subtracted from a signal representative of the nominal flare height value (FH NOM  which is connected to the second additive input of adder 52 and is represented at block 54). 
     It will be recognized that the arrangement and signal processing described relative to high ground speed unit 42 provides a flare height signal FH H  that can be represented by the equation 
     
         FH.sub.H =FH.sub.NOM -DS.sub.1 (SR+SR.sub.1) 
    
     where, SR represents the value of the adjusted approach sink rate, FH NOM  represents the previously discussed preselected nominal flare height, DS 1  represents the rate of change in flare height magnitude per unit change in adjusted approach sink rate for sink rate values in the range SR 1  to SR U , SR 1  represents the value of adjusted approach sink rate at which the high ground speed region of operation and the nominal ground speed region of operation intersect. That is, the flare height at SR 1  is equal to FH NOM , but each sink rate of magnitude greater than the magnitude of SR 1  results in a flare altitude greater than FH NOM . 
     Basically, the signal processing implemented by the invention when the aircraft is executing an automatic landing under relatively low ground speed conditions is identical to the above-discussed signal processing for relatively high ground speed automatic landings. Specifically, identical processing steps can be employed using the magnitude of SR 2  in place of SR 1 . Further, in situations such as that shown in FIG. 1 where the rate of increase in flare height per unit increase in the magnitude of sink rate in the low ground speed range differs from the rate of flare height increase in the high ground speed range, it also is necessary to utilize a constant value DS 2  that differs from the predetermined constant DS 1  used in the low ground speed signal processing. 
     Since the signal processing for the two ground speed ranges incorporates the same processing steps, low ground speed unit 56 of FIG. 2 is depicted as being identical in topology to high ground speed unit 42. In the depicted arrangement, low ground speed unit 56 includes an adder 58 having its input terminals connected for receiving the adjusted approach sink rate signal and a signal representative of the magnitude of SR 2  (indicated at block 60). As was previously mentioned and as can be seen in FIG. 1, SR 2  is the sink rate that marks the boundary between the nominal and low ground speed ranges. A multiplier 62 multiplies the signal provided by adder 58 by the previously mentioned predetermined constant DS 2  (indicated at block 64) and supplies a signal representative of that product to the subtractive input of an adder 66 that has an additive input connected for receiving a signal representative of the nominal flare height FH NOM  (indicated at block 68). Thus, when an automatic landing is made under low ground speed conditions, a flare height FH L  is provided that corresponds to the equation 
     
         FH.sub.L =FH.sub.NOM =DS.sub.2 (SR+SR.sub.2) 
    
     where, SR represents the value of the adjusted approach sink rate and the remaining quantities correspond to the above-described predetermined values of sink rate and desired rate of increase in flare height per unit increase in magnitude of sink rate. 
     The signal processing of the invention that determines whether an automatic landing is being executed under nominal ground speed, high ground speed or low ground speed conditions is illustrated in FIG. 2 by a sink rate detector 70 which operates two switches 72 and 74. The wiper of switch 74 is connected to a limiter 78, which ensures that the flare height signal provided by the invention cannot exceed the maximum flare height FH MAX  and cannot be less than the minimum flare height FH MIN . Thus, as is shown in FIG. 1, the flare height value for all sink rates of magnitude greater than SR U  will be FH MAX  and the flare height for all magnitudes of adjusted approach sink rate less than SR L  will be equal to FH MIN . As is indicated in FIG. 2, sink rate detector 70 operates switch 74 so that limiter 78 will receive the ground speed flare height signal FH L  from low ground speed unit 56 when the magnitude of the adjusted approach sink rate signal is less than SR 2 . For all adjusted approach sink rates of magnitude greater than or equal to SR 1 , switch 74 interconnects limiter 78 with the wiper of switch 72. As is indicated in FIG. 2, when the magnitude of the adjusted approach sink rate is greater than SR 1 , sink rate detector 70 operates switch 72 so that the flare height signal FH H  supplied by high ground speed unit 42 is supplied to limiter 78 via switch 74. 
     When the magnitude of the adjusted approach sink rate is neither less than SR 2  nor greater than SR 1 , the aircraft is executing an automatic landing within the nominal ground speed range of FIG. 1. Under such conditions, a signal representative of the nominal flare height FH NOM  (indicated at block 80 in FIG. 2), is supplied to limiter 78 via switches 72 and 74. 
     In at least some automatic landing systems, the height at which the flare maneuver is initiated is employed to establish open loop gains and/or feedback compensation in control laws that are used for controlling the aircraft during the flare maneuver. Thus, in the currently preferred practice of the invention, the flare height established by the invention is stored for use by the automatic landing system during the remaining portion of the landing procedure. This storage of the flare height initiation value is initiated is indicated in FIG. 2 by a latch circuit 82, which includes a switch 84 and a memory location 86 (i.e., a suitable portion of the automatic landing system memory). As is indicated in FIG. 2, switch 84 is operated upon initiation of the flare maneuver (i.e., &#34;flare engage&#34;). Prior to initiation of flare, the flare height signal provided by limiter 78 is coupled to memory location 86 via switch 84, i.e., memory location 86 is updated with each iteration of the above-described signal processing. When the aircraft reaches the flare height altitude, switch 84 is activated so that the height at which flare was initiated remains stored in memory location 86. 
     It will be recognized by those skilled in the art that various changes and modifications can be made in the methods and apparatus described herein without departing from the scope and spirit of the invention. For example, in many situations, the glideslope signal that is used in generating the adjusted approach sink rate signal can be set to a nominal value such as 3° (which corresponds to the nominal glide path for currently available landing facilities). Further, although discussed herein in terms of specific control block diagrams, it will be recognized by those skilled in the art, that the disclosed signal processing (and equivalent processing) can be implemented in various ways, including a wide range of specifically configured logic circuits and by a variety of programs that control the operation of digital automatic landing systems, autopilots and flight control systems.